Floating body flowmeters of the type designated above are primarily used to register volume flows, that is to say flow rates, in closed pipelines and, because of their simplicity and ruggedness, have found wide application in particular in the chemical and process industry but also in medicine and natural sciences. Floating body flowmeters may be divided into devices in which the floating body interacts with a conical measuring tube and devices in which a conical floating body interacts with a fixed orifice plate arranged in a fixed location with respect to the measuring tube.
The general measuring principle of floating body flowmeters is that, as a result of the weight of the floating body, the buoyancy force of the floating body and the force of the flow of the substance to be measured, a force equilibrium is established. In the event that the flow rate through the measuring tube varies, the force of the flow acting on the floating body changes accordingly, so that the force equilibrium changes, which is changed in a correspondingly varied vertical position of the floating body in the conical measuring tube or in a changed vertical position of the conical floating body in an orifice plate.
The vertical position is a measure of the flow rate. The measuring range of a floating body flowmeter is determined by the geometric dimension and shape of the floating body, of the measuring tube or of the orifice plate, in particular the taper of the measuring tube or of the floating body, but also by the density of the material of which the floating body consists. The present invention is dedicated to the last-named aspect.
DE 39 22 609 A1 reveals a floating body flowmeter which makes use of a conical floating body in combination with an orifice plate. The geometry of the floating body is such that the smallest cross section that acts in the annular measuring gap is located in the central part of the floating body, the length of the floating body without guide elements being equal to or greater than twice the displacement of the floating body, and the upper and lower half optionally tapering differently toward the center of the floating body. Thus, by means of simply rotating the floating body through 180°, a choice can be made between two different measuring ranges. The choice of the measuring range depends in particular on the viscosity of the substance to be measured flowing through the measuring tube and its flow rate. The measuring tube is transparent here, so that the vertical position of the floating body, representing the current flow rate, can be perceived or registered optically from the outside.
DE 34 11 156 A1 reveals a further generic floating body flowmeter, here on the principle of a conical measuring tube in combination with a normal floating body. Accordingly, the floating body is configured so as to correspond to the purpose and can move freely, at least in the flow direction. The substance to be measured and whose flow rate is to be measured flows through the vertically arranged measuring tube counter to the force of gravity, that is to say substantially from bottom to top. Together with the measuring tube, therefore, the floating body forms a throttling point which widens toward the top. The density of the floating body must also be greater than the substance to be measured flowing through the measuring tube. Depending on the flow rate, in the steady-state the floating body is adjusted to a specific vertical position in the measuring tube, which results from the equilibrium between the hydrodynamic force caused by the flow and transferred to the floating body and the difference of the weight or buoyancy forces of the floating body. The taper of the measuring tube, which, in the lower region of the measuring tube, has the effect of a lower diameter than in the upper region of the measuring tube, varies the size of the throttling point along the vertical position of the floating body, so that the result is a mathematical relationship between the vertical position of the floating body and the flow rate through the measuring tube.
In the embodiment specified in this prior art, the measuring tube is opaque and the vertical position of the floating body located on the inside is registered inductively. For this purpose, a plurality of magnetic field sensors placed at a distance from one another are arranged on the outside of the conical measuring tube and respond to the magnetic field of a permanent magnet contained in the floating body. An evaluation device connected downstream supplies a position signal which indicates the height of the magnet and therefore the height of the floating body within the measuring tube.
In the floating body flowmeters of the type described above, suitable floating bodies are usually chosen depending on the viscosity and temperature of the substance to be measured and on the flow range to be measured. In this case, in particular, the weight of the floating body resulting from the density of the material used plays a decisive role. For example, at high flow rates and/or with low viscosities of the substance to be measured, relatively heavy floating bodies are used, whereas at low flow rates and/or with high viscosities of the substance to be measured, lightweight floating bodies are used. Thus, the measuring range of a floating body flowmeter can be adjusted in a simple way via the density of the floating body.
However, the disadvantage here is that, given a predefined geometric shape of the floating body, in order to achieve the desired weight a material would have to be considered which, on the other hand, however, is not chemically compatible with the substance to be measured. Therefore, if the flow rate of aggressive substances to be measured is to be determined by means of a floating body flowmeter, then the material selection of the floating body also influences the chemical resistance to the substance to be measured.
In these cases of multiple dependency in the selection criteria, it is normally necessary to fall back on materials such as platinum alloys, titanium alloys and the like. The additional condition that the material used for the floating body must also be resistant to the substance to be measured has a limiting effect on the number of suitable materials. Thus, complicated materials which meet the existing conditions are normally required.